This invention relates to a method and apparatus for identifying and recording material deformation, fracture, delamination or other response to dynamic stress. More particularly, this invention improves on the methods of dynamic moire interferometry by providing a method and apparatus for directing a rapidly pulsed laser toward a diffraction grating affixed to material which is undergoing stress and simultaneously recording resultant moire interferograms. A basic element in the invention is a unique device combining a ruby laser with ten high-repetition rate q-switches so that ten independently q-switched regions each generate over 100,000 pulses per second. By collimating ten beams and combining them into a pulse train lying on a single axis the invention achieves pulse rates and the recording of interferograms an order of magnitude faster than those achieved in the prior art.
Diffraction moire interferometry uses a reflection-type diffraction grating (i.e. a specimen grating) which is fixed to the object under study and illuminated by at least two mutually coherent collimated beams. If the illuminating beams are set at the proper incidence angles, the plus first diffraction order of one illuminating beam and the minus first diffraction order of the other beam coincide in space along a line normal to the specimen.
As a result, interference fringes (sometimes called moire patterns) representing a contour map of in-plane displacements can be observed. Comparison of fringes before and after loading can be used to determine load-induced displacements.
The technique of moire interferometry has been extended to dynamic moire interferometry, which uses a pulsed laser source. (cf. Deason et al., "Diffraction Moire: The Dynamic Regime," SPIE O-E Lase 87, Los Angeles, January 1987, published by SPIE vol. 746, p. 152) This technique enables the study of dynamic events, effectively freezing stress wave motion and recording its interaction with a material and especially with flaws in a material.
Ruby lasers, which require only approximately 500 nanoseconds to obtain population inversion, are the dominant light source in dynamic interferometry. Pulsing of the output of a ruby laser has long been known in the prior art and pulse rates over 100,000 pulses per second have been achieved. However, none of the prior art pulsed lasers have met the needs in dynamic moire interferometry for an even faster train of high energy pulses with high coherence length.
It is therefore a primary object of this invention to provide a lasing medium coupled with a multiple q-switch which will produce high energy pulses with high coherence length.
In the accomplishment of the foregoing object, it is another important object of this invention to provide a device which will combine said high energy pulses into a single, collimated pulse train.
It is another important object of this invention to provide a moire interferometer which will utilize a high energy pulse train for the generation of diffraction moire interferograms at rates on the order of 1 MHz. It is a further object of this invention to provide a device which will synchronize pulsing of the lasing medium with the occurrence of experimental events, and with the recording of those events in moire interferograms.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.